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Eric Adams · 03-05-2013, 02:46 PM

Hi Guys,
I've got a 48-482 that I received not working (just static) a few months ago. All caps and any questionable resistors have been replaced. All tubes have been double checked. It has been a fill in project. So far I'm just concerned with AM. I got into it again and this is what I found:
I get nice loud static with any type of antenna. Following the directions for alignment I am able to get a good tone at 455Khz and was able to align it. That's as far as it goes. Still static on any dial position. I tried to get a tone at 1000Khz so I set my sig generator and set the radio dial to 1000. Nothing. I tried it again but had the antenna wire connected to one antenna input. This is when the crazy stuff started. Instead of a tone from my sig gen, I picked up a local radio station. As I turned the dial on the signal gen, I was able to pull in all my local AM stations.
What do you think?

Thanks,
Eric

Ron Ramirez · 03-05-2013, 04:48 PM

I think the oscillator is dead. Check the coils. How about any mica caps in the oscillator circuit? These can and do go bad.

Eric Adams · 03-05-2013, 05:03 PM

Thanks Ron, I'll take a look. Is it best to just use an ohm meter to check the coils for continuity? Should they be disconnected from the circuit?

jerryhawthorne · 03-05-2013, 09:34 PM

Eric, the ohm meter is just fine and checking in circuit is usually fine as well. All coils should show a very low resistance.
Jerry

Eric Adams · 03-05-2013, 11:12 PM

Hi Jerry,

All of the coils are .1 or 0 ohms. My DVOM has a setting for checking caps but it doesn't appear to be very accurate. Should I be looking for an open, short or what on these mica caps?

Thanks,
Eric

Ron Ramirez · 03-06-2013, 07:48 AM

Either open, short, or leakage.

Eric Adams · 03-06-2013, 01:00 PM

Ron,
What's the best way to test for leakage?

Thanks,
Eric

Ron Ramirez · 03-06-2013, 01:15 PM

There are - or should I say were - capacitor testers on the market that would test for leakage, shorts, etc.

If it were me, I'd just replace the D**n things and be done with it. Others will chime in with their opinions...hopefully. Icon_smile

morzh · 03-06-2013, 01:25 PM

Ceramic cap exposed to a reasonably high constant (not AC) voltage should exhibit practically zero current. The insulation resistance is typically in hundreds to thousands MOhms.
The datasheet will specify that for every type. Anything below that is leakage. An egregious leakage basically turns a cap into a resistor, which you do not want.

jerryhawthorne · 03-06-2013, 01:47 PM

Long ago, a mentor told me the best way to test wax caps and electrolytic caps was to remove them, hold them over an empty metal trash can and drop them. If they make a nice noise when they land, they were bad. Your coils look good to go.
Jerry

morzh · 03-06-2013, 02:17 PM

And I can still make a fairly accurate voltage measurement holding the leads to my tongue.
The only problem is the range is limited to 10-12V. Icon_lol

codefox1 · 03-06-2013, 02:23 PM

Agreed, capacitors, toss 'em. Easy and cheap enough to replace carbon resistors at the same time. Long term reliability and performance results.

Eric Adams · 03-06-2013, 02:41 PM

Every electrolytic and wax cap was replaced as the first step. Are you suggesting that I just replace EVERY mica cap?

morzh · 03-06-2013, 02:49 PM

No, micas can be left in.

Papers and lytics have to go.

Eric Adams · 03-06-2013, 03:28 PM

They are all long gone. Any idea how to address my original problem?

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